Thinking takes place through the aggregate action of billions of simple elements--cells called neurons--that are wired up in an extremely complicated way. How Dow Jones Industrial averages react is also a composite aggregate effect of the thinking of millions of investors. As a result neither can be predicted. The comparison goes beyond superficial impact. We have to look into collective conscience as a result of neuronal interactions. Neurons conduct signals in the form of tiny electrical impulses, known as spikes. Messages travel from one neuron to another as pulses of chemicals that are released at specialized junctions, or synapses; there are trillions of such junctions in the human brain. How and when synapses relay messages between neurons is crucially important in controlling mental activity. Moreover, neuroscientists believe that learning occurs through a change in the strength of certain synaptic connections. A frequently-used synapse becomes stronger, whereas an infrequently used one may grow weaker over time. Similarly, the direction Dow Jones takes or the collective conscience of a society evolves in many different directions. Neurons actually have numerous electrically-active components in the incoming branches. These active components, which include the NMDA receptor, a protein that spans the neuronal membrane, modify the effect of incoming messages. For example, the active components ensure that spikes received at synapses that are adjacent to one another carry more weight than spikes received at widely-separated synapses. Computer simulations show that active elements probably multiply the influence of adjacent synapses, rather than merely adding them together as the traditional neurologists had supposed. This finding adds a layer of complication to the picture of how the brain works.

And the story gets still more involved since the theory that the timing of individual spikes is unimportant turns out to be quite wrong. Spikes, once initiated in a neuron, do not propagate only in the "forward" direction--that is, toward the synapses that relay outgoing messages. The effect that these back-propagated spikes have on the active components of the brain is not clear. The precise order in which one spike arrives at a synapse and another one back-propagates to the receiving neuron greatly influences the subsequent strength of a synapse. If the back-propagated spike arrives first, the synapse is weakened; conversely, if the back-propagated spike arrives second, the synapse is intensified. This unexpected phenomenon might specifically boost the synapses that are conveying messages while suppressing random or unimportant signals. The brain, then, is quite unlike a digital computer in its basic underpinnings. And it is this dissimalarity between hujman brain and the digital computer that makes the simulation of Wall Street behavior so difficult—in this case impossible. It also explains why the collective conscience of people can not be predicted; it is evolutionary, the direction not known.